The Synthesis of Ba‐ and Fe‐ Substituted CsAlSi₂O₆ Pollucites

Abstract

Barium‐substituted CsAlSi2O6 pollucites, CsxBa(1−x)/2AlSi2O6, and barium‐ and iron‐substituted pollucites, CsxBa(1−x)/2AlxFe1−xSi2O6 and CsxBa1−xAlxFe1−xSi2O6 were synthesized with 1 ≥ x≥ 0.7 using a hydrothermal synthesis procedure. Rietveld analysis of X‐ray diffraction data confirmed the substitution of Ba for Cs and Fe for Al, respectively. The crystallographic analysis also describes the effects of three different types of pollucite substitutions on the pollucite unit cell: Ba2+ for Cs1+ cation results in little effect on cell dimensions, intermediate concentrations of Ba2+ and Fe3+ substitution result in net minor expansion due to Fe3+ addition, and large Ba and Fe substitutions result in overall framework contraction. Elemental analysis combined with microscopy further supports the phase purity of these new phases. These materials can be used to study the stability of CsAlSi2O6 as a durable ceramic waste form, which could accommodate with time Cs and its decay product, Ba. Furthermore, success in iron substitution for aluminum into the pollucite lattice predicts that redox charge compensation for Cs cation decay is possible.

Crystal Structure and Thermodynamic Stability of Ba/Ti‐Substituted Pollucites for Radioactive Cs/Ba Immobilization

“…This is probably due to the rigidity of the titanosilicate framework, where the enclosed Cs + and Ba 2+ play a secondary role on the dimensions of the pollucite structure. Similar behavior has been observed in Ba‐substituted CsAlSi 2 O 6 pollucites . Other factors, especially the differences among these samples in Cs/Ba ratio, order–disorder of Cs + , Ba 2+ and vacancies and of framework Si 4+ and Ti 4+ , and the amount of cavity O 2− (for the two pollucite phases in the Cs x Ba 1− x TiSi 2 O 7−0.5 x samples, which show slight stoichiometric departures from the two‐Cs‐to‐one‐Ba substitution) may be also responsible.…”

“…Similar behavior has been observed in Ba-substituted CsAlSi 2 O 6 pollucites. 16 Other factors, especially the differences among these samples in Cs/Ba ratio, order-disorder of Cs + , Ba 2+ and vacancies and of framework Si 4+ and Ti 4+ , and the amount of cavity O 2À (for the two pollucite phases in the Cs x Ba 1Àx TiSi 2 O 7À0.5x samples, which show slight stoichiometric departures from the two-Cs-to-one-Ba substitution) may be also responsible. It appears that the effects due to these factors may cancel out to some extent, contributing to the similarities in unit cell dimensions.…”

“…Thus, some order/disorder involving Cs + , Ba 2+ and vacancies may occur over the cavities sites (although X-ray cannot effectively distinguish Cs and Ba because of their similar atomic numbers), as probably occurred in Ba-substituted CsAlSi 2 O 6 pollucites. 16 In addition, to compensate the positive charge surplus arising from the substitution of Al 3+ by Ti 4+ in CsAlSi 2 O 6 , extra O 2À anions are incorporated into the Ti-substituted structure. X-ray absorption spectroscopy analysis reveals a fivefold coordination of Ti 4+ , 17 implying that the excess O 2À anions are mainly bonded to Ti 4+ (forming a titanyl [TiO 5 ] group).…”

(3+1)-Incommensurately modulated crystal structure of Cs3ScSi6O15

“…Due to its short half-life of about 30 years, 137 Cs has high activity and is still the main source of beta-radiation after the nuclear disasters related to the accidents in the power plants of Chernobyl and Fukushima Dai-ichi (Povinec et al, 2013;Steinhauser et al, 2014). Several phases have been proposed in the past as potential hosts for the immobilization of radioactive caesium including porous framework structures such as pollucite (CsAlSi 2 O 6 ), CsAlSiO 4 or CsAlSi 5 O 12 (Rodriguez et al, 2013;Gatta et al, 2008;Fisch et al, 2008) or mica-related compounds such as Cs-annite [CsFe 3 (AlSi 3 )O 10 (OH) 2 ] (Klika et al, 2006).…”

A Cesium Rare‐Earth Silicate Cs3RESi6O15 (RE=Dy–Lu, Y, In): The Parent of an Unusual Structural Class Featuring a Remarkable 57 Å Unit Cell Axis

“…Cesium metal silicates important as radioactive Cs + ion separation and storage materials. [8,9] All the known anhydrous cesium rare earth silicates Cs3EuSi6O15, [10] Cs3DySi6O15, [11] Cs1.86K1.14DySi6O15, Cs1.6K1.4SmSi6O15, [7] and Cs3ScSi6O15, [12] adopt the Cs3RESi6O15 formulation, but each has a different structure, all of which seem unique to the field. Typically the compounds are prepared either using a molten flux or a high temperature hydrothermal method.…”